Heavy metal fractions and ecological risk assessment in sediments from urban, rural and reclamation-affected rivers of the Pearl River Estuary, China

- Higher heavy metals contents were found in urban river sediment.
- Cd posed a medium-to-high potential ecological risk to the river environment.
- Reducible and residual fractions were the major geochemical phases of heavy metals.
- Cd, Cu and Zn exhibited moderately to heavily polluted in river sediment.
- Sediment organic carbon affected the distribution of metals in organic fraction.

Rapid urbanization and reclamation processes in coastal areas have resulted in serious pollution to the aquatic environment. Less is known on the geochemical fractions and ecological risks in river sediment under various human activities pressures, which is essential for addressing the connections between heavy metal pollution and anthropogenic influences. River sediments were collected from different landscapes (i.e., urban, rural and reclamation areas) to investigate the impacts of urbanization and reclamation on the metallic pollution levels and ecological risks in the Pear River Estuary of China. Results showed that Cd, Zn and Cu with high total contents and geoaccumulation index (Igeo) were the primary metals in the Peal River sediments. Generally, urban river sediments, especially the surface sediment layer (0-10 cm), exhibited higher metallic pollution levels. As for geochemical fractions, reducible and residual fractions were the dominant forms for six determined metals. And the percentage of heavy metals bound to Fe-Mn oxides decreased with increasing soil depth but the reverse tendency was observed for residual fractions. Compared with rural river sediments, heavy metals were highly associated with the exchangeable and carbonate fractions in both urban and reclamation-affected river sediments, suggesting that anthropogenic activities mainly increased the active forms of metals. Approximately 80% of Cd existed in the non-residual fraction and posed medium to high ecological risk according to the risk assessment code (RAC) values. The redundancy analysis (RDA) revealed that both urbanization and reclamation processes would cause similar metallic characteristics, and sediment organic matter (SOC) might be the prominent influencing factor.